The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. However, these advances have increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC processing and manufacturing are needed. In the course of integrated circuit evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased.
To enhance the performance of ICs, metal gate transistors have been used in recent years. However, conventional metal gate transistors may suffer from an N/P boundary effect. In more detail, when a P-type metal gate transistor borders an N-type metal gate transistor, contamination may occur through metal diffusion across the boundary between the P-type and N-type metal gate transistors. Such contamination may degrade the threshold voltage (Vt) of the metal gate transistors. Moreover, as device sizes continue to shrink, limitations in current lithography technology may exacerbate the undesirable Vt shifting issue discussed above, thereby further degrading the performance of conventional metal gate transistors.
Therefore, while existing methods of fabricating metal gate transistors have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect.